An electric power steering apparatus (EPS) which assists and controls a steering system of a vehicle by means of a rotational torque of a motor, applies a driving force of the motor as a steering assist torque (an assist torque) to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key (IG) signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command on the basis of a steering torque Ts detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for the EPS by means of a voltage control command value Vref obtained by performing compensation or the like to the current command value.
Moreover, the steering angle sensor 14 is not essential, it does not need to be provided, and it is possible to obtain the steering angle from a rotation sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 100 exchanging various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed V from the CAN 100. Further, it is also possible to connect a non-CAN 101 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN 100 to the control unit 30.
The control unit 30 mainly comprises an MCU (including a CPU, an MPU and so on), and general functions performed by programs within the MCU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 (or from the CAN 100) are inputted into a current command value calculating section 31 that calculates a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a motor current supplied to the motor 20 on the basis of the inputted steering torque Ts and vehicle speed V and by using an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation ΔI (=Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation ΔI is inputted into a proportional integral (PI) control section 35 for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 uses field effect transistors (FETs) as driving elements and is comprised of a bridge circuit of FETs.
A compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 34-3 and an inertia 34-2 at an adding section 34-4, further adds the result of addition performed at the adding section 34-4 with a convergence 34-1 at an adding section 34-5, and then outputs the result of addition performed at the adding section 34-5 as the compensation signal CM.
In such an electric power steering apparatus (EPS), since stability and responsiveness of an operation are contrary characteristics, compatibility of both characteristics becomes a problem. In particular, in the case of considering developing the EPS to a driving support apparatus, it is desired to stabilize a vibration mode of a two-inertia system consisting of steering wheel inertia and motor inertia which are constituting the EPS, and suppress a vibration without deteriorating responsiveness.
There is, for example, a control apparatus described in the publication of Japanese Patent No. 3493806 B2 (Patent Document 1) as such an apparatus improving the stability of the vibration mode. The control apparatus described in Patent Document 1 improves stability and responsiveness of a control system by comprising a stability compensator that operates so as to counteract a peak of a resonance frequency of a resonance system consisting of an inertia element and a spring element which constitute an EPS. The stability compensator performs pole-and-zero cancellation in order to counteract the peak of the resonance frequency.
A control unit described in the publication of Japanese Patent No. 3385763 B2 (Patent Document 2) improves stability when a steering wheel is located at the center (stability at the on-center). In the vicinity of the on-center, that is, in the case that a vehicle travels straight, quick steering is not performed, and there are time that correction of a current command value is not performed sufficiently, so that the control unit detects the straight traveling state of the vehicle, and improves the stability in the vicinity of the on-center by adding a correction value to the current command value in the straight traveling state.